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<br /> <br />Active vs. Inective Fans <br /> <br />In the case of geologically younger, or active fans, the natural <br />processes which, over time, formed the fan continue to act <br />upon its suriace in a somewhat random fashion. Thus, the <br />entire fan surface represents a potential site for flood, sedi- <br />ment and debris deposition and scour, and is a dangerous <br />location upon which to build. <br /> <br />In geologically older regions, alluvial fan processes tend to <br />follow more predictable patterns, and f1oodflows may remain <br />within defined channels as they discharge downslope. Fans or <br />portions of fans which are no longer subject to these flows are <br />considered inactive, and may be regarded as less hazardous <br />construction sites. <br /> <br />Regardless 01 whether a fan is active, inactive, or both, there <br />is always some degree of unpredictability of the Ilood hazard <br />on all fans. <br /> <br />A Wo,d About Deb"s HSZ8,ds. . . <br /> <br />Debris flows can result from the presence of a large percent- <br />age (up to 70-90% of flow by weight) of fine sediment such as <br />si~ and clay in steeply-flowing floodwaters. This enables the <br />muddy flow to transport sand, gravel, boulders, and dislodged <br />timber and brush from the mountain watershed onto a fan's <br />suriace. Conditions favoring the formation of debris flows are: <br />available unconsolidated silt, clay and larger rock in the basin <br />watershed (due to minimal vegetation), heavy or sustained <br />rainfall in the basin, and the presence of steep basin and fan <br />slopes. Fans which have been created from repealed debris <br />flow activity are called debris fans, and are composed of <br />deposits of rock, soil and vegetation from the upstream water. <br />shed. Debris fans are found in areas where mountain systems <br />are subject to tectonic forces of uplift, Colorado, Utah, Wyo- <br />ming, Montana, and parts of California, Nevada and Arizona <br />contain the bulk of debris fans and flows. <br /> <br /> <br />FigUf9 10. <br />Development on a debris fan in Georgetown, Colorado. Nole th9 structures' proximity to the steep mountain sJop8 abo~. (Courtesy <br />of thfl Colorado G801oglcal Survey) <br /> <br />6 <br /> <br />- <br />